This invention relates to analog/digital converters and in particular to a method for statistically measuring the differential linearity of such devices.
There currently exists a variety of methods for measuring the differential linearity of an analog/digital converter. One method is to apply a known function such as a slow ramp to the input of the analog/digital converter, convert the analog/digital converter output back to the analog with a digital/analog converter and subtract it from the input. The difference from the subtractor is a series of ramps. If the digital/analog converter used in the conversion from digital to analog is perfect, the amplitudes of the ramps provide a measure of the differential linearity. This method requires a very good digital/analog converter in the test equipment.
A second method applies a "DC" voltage to the input of the analog/digital converter. The output of the analog/digital converter is examined. The "DC" input is varied slightly until the analog/digital converter output word changes between two adjacent output words. The "DC" input voltage is measured with an accurate digital voltmeter and is the DC threshold of an output word. The "DC" is changed to find the threshold of the next output word and so on until all thresholds are measured.
The two above methods require precision equipment. The first requires a precision digital/analog converter. The second requires a high precision fast reading digital voltmeter.
The need for a different approach to measurement becomes necessary when it is required to measure the differential linearity of an analog/digital converter which has "correction loops" that constantly change the offset of the analog/digital converter under test. Although the details of the analog/digital converter design are unimportant, a significant point to consider is that the "threshold method" of measurement described above is colored by the correction loop activity. The other method of measurement (re-conversion and subtraction) is subjective in that it requires a trained eye to interpret the amplitudes of the ramps.
The foregoing and other state-of-the-art approaches to measuring the differential linearity of analog/digital converters indicate the need for a new method of providing accurate linearity measurements of analog/digital converter that can be implemented without the requisite of precision calibrated equipment and/or the subjective interpretation of data. It is also desirable that the measurements be immune to noise on the input waveform and to DC drifts in the analog/digital converter. The present invention is directed toward satisfying that need.